Ingredients

Cooking spray
3/4 cup all-purpose flour
1/2 cup unsweetened cocoa
1/4 teaspoon salt
1/2 teaspoon baking powder
4 tablespoons unsalted butter, at room temperature
2/3 cup sugar
1 large egg
2 egg whites
1 teaspoon vanilla extract
2 tablespoons brewed strong coffee, cooled to room temperature
1/2 cup nonfat Greek yogurt
1/4 cup mini semisweet chocolate morsels
1/4 cup chopped walnuts (optional)